Investments - Schedule of Unrealized Investment Losses by Class of Investment (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 4,268,097	$ 11,450,679
Unrealized Loss, Less than Twelve Months	(238,604)	(1,846,999)
Fair Value, Twelve Months or Longer	6,786,036	433,328
Unrealized Loss, Twelve Months or Longer	(1,419,748)	(190,295)
Fair Value	11,054,133	11,884,007
Unrealized Loss	(1,658,352)	(2,037,294)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	4,108,960	11,121,941
Unrealized Loss, Less than Twelve Months	(224,533)	(1,796,781)
Fair Value, Twelve Months or Longer	6,522,450	357,361
Unrealized Loss, Twelve Months or Longer	(1,344,978)	(158,773)
Fair Value	10,631,410	11,479,302
Unrealized Loss	(1,569,511)	(1,955,554)
Investment Grade | U.S. Government direct, guaranteed, and government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	275,114	349,887
Unrealized Loss, Less than Twelve Months	(17,384)	(38,218)
Fair Value, Twelve Months or Longer	84,691	3,424
Unrealized Loss, Twelve Months or Longer	(10,098)	(750)
Fair Value	359,805	353,311
Unrealized Loss	(27,482)	(38,968)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	601,542	1,767,624
Unrealized Loss, Less than Twelve Months	(23,575)	(453,149)
Fair Value, Twelve Months or Longer	1,219,137	95,124
Unrealized Loss, Twelve Months or Longer	(403,354)	(52,298)
Fair Value	1,820,679	1,862,748
Unrealized Loss	(426,929)	(505,447)
Investment Grade | Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	15,491	6,297
Unrealized Loss, Less than Twelve Months	(145)	(201)
Fair Value, Twelve Months or Longer	25,555	25,134
Unrealized Loss, Twelve Months or Longer	(11,137)	(12,505)
Fair Value	41,046	31,431
Unrealized Loss	(11,282)	(12,706)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	3,191,080	8,937,976
Unrealized Loss, Less than Twelve Months	(182,733)	(1,299,990)
Fair Value, Twelve Months or Longer	5,149,436	225,719
Unrealized Loss, Twelve Months or Longer	(914,935)	(90,785)
Fair Value	8,340,516	9,163,695
Unrealized Loss	(1,097,668)	(1,390,775)
Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,355,645	2,837,918
Unrealized Loss, Less than Twelve Months	(92,771)	(426,132)
Fair Value, Twelve Months or Longer	1,482,482	109,784
Unrealized Loss, Twelve Months or Longer	(308,161)	(42,173)
Fair Value	2,838,127	2,947,702
Unrealized Loss	(400,932)	(468,305)
Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	304,897	1,088,219
Unrealized Loss, Less than Twelve Months	(14,438)	(116,272)
Fair Value, Twelve Months or Longer	474,873	21,636
Unrealized Loss, Twelve Months or Longer	(77,678)	(6,268)
Fair Value	779,770	1,109,855
Unrealized Loss	(92,116)	(122,540)
Investment Grade | Corporates | Energy
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	319,199	855,853
Unrealized Loss, Less than Twelve Months	(11,489)	(91,755)
Fair Value, Twelve Months or Longer	332,106	0
Unrealized Loss, Twelve Months or Longer	(58,433)	0
Fair Value	651,305	855,853
Unrealized Loss	(69,922)	(91,755)
Investment Grade | Corporates | Other corporate sectors
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,211,339	4,155,986
Unrealized Loss, Less than Twelve Months	(64,035)	(665,831)
Fair Value, Twelve Months or Longer	2,859,975	94,299
Unrealized Loss, Twelve Months or Longer	(470,663)	(42,344)
Fair Value	4,071,314	4,250,285
Unrealized Loss	(534,698)	(708,175)
Investment Grade | Collateralized debt obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	0
Unrealized Loss, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	0	0
Unrealized Loss	0	0
Investment Grade | Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	25,733	60,157
Unrealized Loss, Less than Twelve Months	(696)	(5,223)
Fair Value, Twelve Months or Longer	43,631	7,960
Unrealized Loss, Twelve Months or Longer	(5,454)	(2,435)
Fair Value	69,364	68,117
Unrealized Loss	(6,150)	(7,658)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	159,137	328,738
Unrealized Loss, Less than Twelve Months	(14,071)	(50,218)
Fair Value, Twelve Months or Longer	263,586	75,967
Unrealized Loss, Twelve Months or Longer	(74,770)	(31,522)
Fair Value	422,723	404,705
Unrealized Loss	(88,841)	(81,740)
Below Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	0
Unrealized Loss, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	0	0
Unrealized Loss	0	0
Below Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	159,137	328,738
Unrealized Loss, Less than Twelve Months	(14,071)	(50,218)
Fair Value, Twelve Months or Longer	252,006	65,093
Unrealized Loss, Twelve Months or Longer	(73,962)	(29,904)
Fair Value	411,143	393,831
Unrealized Loss	(88,033)	(80,122)
Below Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	79,716	120,377
Unrealized Loss, Less than Twelve Months	(9,059)	(18,901)
Fair Value, Twelve Months or Longer	106,971	38,348
Unrealized Loss, Twelve Months or Longer	(43,403)	(17,283)
Fair Value	186,687	158,725
Unrealized Loss	(52,462)	(36,184)
Below Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	8,265	27,722
Unrealized Loss, Less than Twelve Months	(662)	(3,173)
Fair Value, Twelve Months or Longer	20,166	0
Unrealized Loss, Twelve Months or Longer	(1,801)	0
Fair Value	28,431	27,722
Unrealized Loss	(2,463)	(3,173)
Below Investment Grade | Corporates | Energy
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	14,480
Unrealized Loss, Less than Twelve Months	0	(2,182)
Fair Value, Twelve Months or Longer	34,289	20,075
Unrealized Loss, Twelve Months or Longer	(10,416)	(7,986)
Fair Value	34,289	34,555
Unrealized Loss	(10,416)	(10,168)
Below Investment Grade | Corporates | Other corporate sectors
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	71,156	166,159
Unrealized Loss, Less than Twelve Months	(4,350)	(25,962)
Fair Value, Twelve Months or Longer	90,580	6,670
Unrealized Loss, Twelve Months or Longer	(18,342)	(4,635)
Fair Value	161,736	172,829
Unrealized Loss	(22,692)	(30,597)
Below Investment Grade | Collateralized debt obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	0
Unrealized Loss, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	0	0
Unrealized Loss	0	0
Below Investment Grade | Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	0
Unrealized Loss, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	11,580	10,874
Unrealized Loss, Twelve Months or Longer	(808)	(1,618)
Fair Value	11,580	10,874
Unrealized Loss	$ (808)	$ (1,618)